The subject matter disclosed herein relates generally to nuclear medicine (NM) imaging systems, and more particularly to methods and systems for determining gamma camera uniformity correction for the NM imaging systems.
NM imaging systems, for example Single Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET) imaging systems, use one or more image detectors to acquire image data, such as gamma ray or photon image data. The image detectors may be gamma cameras that acquire two-dimensional views of three-dimensional distributions of emitted radionuclides (from an injected radioisotope) from a patient being imaged.
In single photon imaging systems, such a planar or SPECT imaging systems, collimators may be placed in front of a scintillation crystal or solid state detector to form an image of a radioactive source and to focus the field of view (FOV) of the detectors. The collimators allow gamma rays aligned with the holes of the collimators to pass through to the detector. These detectors need to be calibrated, including during system manufacture and periodically after installation to ensure proper imaging operation. For example, the detectors are calibrated to provide a uniform sensitivity response across the detector units or output channels.
Specifically, conventional gamma cameras for NM imaging systems with detachable or/and fixed parallel-hole collimators are usually calibrated using a uniform gamma flux from an emission source, which irradiates the detector component (exposes the detector to radioactive emissions) that is sensitive to gamma rays. The uniform gamma flux is usually created by a point source located a far distance from the detector with the collimator detached. Alternatively, a uniform flood source can be used for a detector equipped with an attached collimator.
During the calibration process, the detector response to uniform gamma flux is measured. The count density of the planar image is usually considered a measure of the detector response. Non-uniformity of the planar image count density is caused by variation of detector responses within a system. Accordingly, a gamma-camera uniformity correction map is derived from the acquired planar image count density data to correct for the non-uniformity, often referred to as sensitivity calibration.
If a gamma camera is equipped with a fixed non-parallel-hole collimator (e.g., pinhole, fan-beam, cone-beam collimator), which is not detachable, such as in some clinical conditions, then the requirement for detector irradiation to be a uniform gamma flux becomes very difficult or even impossible to implement.